ORCESTER - Twice the little girl has come to this office for a talk about her private parts, and twice her mother has taken her back home. Nothing.

Helene Murphy is certain she knows what happened to the girl. Murphy has sat beside the 31/2-year-old as she babbled sentences and fractions of sentences about her mother's boyfriend. She has noted the bed-wetting, the reluctance to make eye contact, the compulsive masturbation that has caused chafing.

After 20 years in the field of sexual abuse evaluation, Murphy can tell this is going to be one of the cases that drive her crazy.

''We're talking about a girlwho has much to disclose,'' said Murphy, director of social services at the University of Massachusetts Memorial Medical Center Child Protection Program. ''She's going home at night every night with this guy, because we don't have the information to say, `Grab this guy,' before it goes on and on.''

The current priest scandal has put sexual abuse in the headlines for months, but doctors who evaluate abused children say the increased attention has done little for their beleaguered specialty.

Twenty years since child sexual abuse burst into the public consciousness - and 10 years after several high-profile convictions fell apart - proving suspected sexual abuse has only become more difficult.

According to research, the national number of child sexual abuse allegations declined sharply through the 1990s, along with the number of allegations that have been substantiated. In Massachusetts, the Department of Social Services became far more stringent about corroborating suspicions - the number of cases screened out before investigation rose from 16 percent in 1990 to 40 percent in 1997.

And some medical examiners say they simply require a far higher standard of proof before answering the terrible question of whether abuse occurred.

''We'll diagnose sexual abuse in about 25 percent of cases [in which abuse is suspected]. That probably means some false negatives,'' said Lawrence Ricci, who heads the Spurwink Child Abuse Program in Portland, Maine. ''But it's always a balance. When you're dealing with something of an inexact science, you have to decide which you're willing to accept.

''Gone are the days when we used to say in this field, `If we're going to err, let's err on the side of protecting the child.' It's always a pendulum.''

It is this swing of the pendulum that frustrates Murphy. In a meeting three weeks ago, she said, a representative from the district attorney's office told her that the behavior of the 31/2-year-old girl wasn't enough evidence to merit a prosecutorial investigation. So the child would have to take a place on a long waiting list for psychological evaluation by a therapist specializing in child abuse, who will try to determine what, if anything, happened.

''It's a thankless kind of a thing,'' Murphy said, sighing. ''It's just in my gut and I can't do anything about it, and the child is going home with him.''

Spurred by a TV movie, the silence is broken

Reports of sexual abuse exploded in the 1980s, and with it came optimism that Americans could address the crime as a society.

Jetta Bernier recalls the morning in January 1984 after ABC aired the docudrama ''Something About Amelia,'' in which an upper-middle-class father, played by Ted Danson, had a secret sexual relationship with his daughter. Reports began pouring in through the mail and over the phone, said Bernier, who was then a senior staff member at the National Committee to Prevent Child Abuse. ''The country went wild,'' she said.

In the following years, therapists and social workers around the country attended abuse workshops while victims' advocates plastered bumper stickers swearing their faith in victims.

But just as enormous was the wall of skepticism that followed. Starting in the early 1990s, a series of sensational cases were challenged one after another - the McMartin Preschool case in California, the Little Rascals Day Care case in North Carolina, the Fells Acres Day Care case in Massachusetts - leaving many Americans with the lasting impression that social scientists had whipped up children's claims. Ever since, said Tufts pediatrician Robert Reece, the specialty has been under attack.

''It's considered a soft science. Child abuse is considered by some to be a problem that's been blown out of proportion,'' said Reece, who wrote two textbooks on diagnosing and treating abused children.

Since then, allegations of child sexual abuse have receded. After rising steadily from 1977 to 1992, the national number of substantiated reports turned and declined a remarkable 31 percent by 1998, from 149,800 cases to 103,600, according to a report for the Department of Justice done by the University of New Hampshire.

The number of children reporting that they have been abused has dropped at a similar rate, suggesting that the decline relects a genuine drop, not simply a reluctance on the part of child advocates to report, according to David Finkelhor, who heads the Crimes against Children Research Center at UNH.

In addition, Massachusetts officials have become increasingly cautious about corroborating allegations of sexual abuse. Between 1990 and 1997, the last year for which records are available, the number of sex abuse reports ''screened in'' for investigation in the DSS system has dropped 24 percent. Those allegations that were eventually supported by DSS fell from 44 percent to 33 percent.

For doctors, proving an assault has become harder. Sexual abuse experts who used to assume that conclusive physical evidence of sexual abuse was frequent now believe that it is only present in 5 percent of cases, Ricci said.

In a paper published last year in the journal Child Abuse and Neglect, Ricci and two colleagues dismissed the idea that abuse could be determined from watching for telltale signs in the child's behavior, such as masturbation, concluding that in there was ''no significant relationship'' between abuse and such behavior.

''Our understanding clearly puts the burden of evidence on the history from the child,'' Ricci said.

How far to go to get that history is another matter. In the past, children were interviewed multiple times, privately, by therapists and police. But after Fells Acres, wary prosecutors have turned to the standardized Sexual Abuse Investigation Network interview, in which the child is interviewed only once by a therapist, as a multidisciplinary team of experts watches from behind one-way glass. In cases where there is strong evidence, prosecutors have 10 days to make a determination on whether there has been an assault.

But often, strong evidence is hard to come by. At the Child Protection Program in Worcester, 10 percent to 15 percent of the children brought in readily disclose that sexual abuse took place, said pediatrician Christine Barron, who oversees the unit. Last year, Barron delivered a positive diagnosis in 43 percent of cases and a negative diagnosis in 26 percent of cases, she said. The remaining 30 percent of cases fall into the category of ''unclear'' and are referred for ongoing counseling, she said.

The difficult path toward the truth

Somewhere in that gray area is Murphy's patient. Murphy would like to see her take a ''psychological sexual abuse evaluation,'' in which the child would meet one-on-one with a therapist regularly over a period of weeks or months.

''There are very few children you can't get anything out of,'' she said. ''Set up that same child with a nurturing parent-type person and the child begins to build up a relationship with that person, and you'd be amazed at what can come out of a child's mouth.''

''Those are the kids that are sitting on the waiting list for three months,'' said Andrea Vandeven, medical director of the child abuse team at Children's Hospital, who estimates that between a third and a half of the 200 to 250 sexual abuse cases she sees every year are not clear-cut enough for prosecution. When prosecutors demur, families are referred to therapists for long-term counseling. At that point, many cases simply recede, she said.

''If you can't get the child evaluated right away, it dissipates,'' she said. ''The family may barrage the child with questions, which causes the child to shut down.''

And often, Murphy is left to worry about the information that didn't quite come out.

''She talked a little about `Uncle Dan,''' Murphy said, using a pseudonym ''To even get a sentence like that, you're putting words together. The eye contact. `I don't want to see Uncle Dan. I don't want to go see Uncle Dan.'''